Population Dynamics of the Lower Kananaskis Lake Bull Trout: 1995-2000 Final Report
Craig J. Mushhens, Fiona D. Johnston, and John R. Post
Alberta Enviornmental Protection in 1991, determined that the number of bull trout in the Lower Kananaskis Lake population had decreased considerably in comparison to historical reports. This lead to the implementation of a catch-and-release fishery for bull trout in the Lower Kananaskis Lake and closure of the Smith-Dorrien Creek and the northwest bay of Lower Kananaskis Lake to angling in the spring of 1992. There are several possible factors that contributed to the decline of the population, including overharvest by anglers, a small minimum size limit (40 cm) and unstable habitat due to fluctuating water levels. Since the fall of 1990, various levels of research have been carried out, by various parties, on the bull trout from 1991 to 1993. A considerable amount of interest has been expressed about the possiblity of developing Lower Kananaskis Lake into a bull trout trophy fishery. To determine the feasibility of such a fishery and to manage it optimally, information about the dynamics and the life-history of this population were required. A second study was implemented in 1995 to help address these questions. The following report presents results from the past six years of this second field study (1995 to 2000) carried out by the University of Calgary with the support of A.C.A. and TransAlta, and also integrates data collected during the previous study from 1991 through 1993, by associates of Alberta Environmental Protection, as documented in Stelfox and Egan (1995). Analysis of the data collected from 1991 through 1993 may differ from that found in Stelfox and Egan (1995).
The results from this six year study confirm that the numbers of spawning adult bull trout in Lowe Kananaskis Lake / Smith-Dorrien Creek have increased dramaticlally since 1992, with over a 20-fold increase from 60 spawners in 1992, to 1370 in 2000. The number of spawners has fluctuated since 1998 due to the occurrence of alternate-year spawning especially by males. This may explain the female skewed sex ratio observed in this system. Although numbers have increased, the number of new spawning recruits has not increased in 1995, and these rates were found to decline with fish size. This suggests that larger fish are allocating more energy to reproduction. This also results in fish of poorer condition. Diet analysis of bull trout caught in Lower Kananaskis Lake indicated that smaller bull trout preferred Mysids and benthic invertebrates, whereas larger fish were increasingly piscivorous. Juveniles that enter the lake have higher growth than in the stream. Results from aging and PIT tag returns suggest that females mature earlier than males at age-6 or age-7 and males mature later at age-7 or age-8. Juvenile downstream migration apprears to coincide with the upstream migration of sdults during their spawning run. Catch-and-release regulations have lowered maximum annual mortality rates from 50% to 5% to 15%. Hooking mortality and illegal harvest contribute to mortality rates with maximum estimates indicating that roughly between 12& and 15% of fish that are caught by anglers experience mortality. Valuable demographic and life history information has been gained about this population that may be used to manage it more effectively, however, a number of questions still remain unanswered.